A number of processes are already known for producing or packing sand molds for foundry purposes. Such processes include embodiments in which compacting of the granular material is caused by the pressure shock of an exothermic reaction of a gas mixture such as in U.S. Pat. No. 3,170,202. In other processes the shock wave caused by releasing the pressure of a high pressure gas is used for compacting a granular material such as in German patent specification No. 1,097,622.
If this shock pressure acts on a loosely distributed mass of granular material, the material will be accelerated and then compressed by subsequent retardation at the pattern-supporting plate.
However, on the back side of the mold (i.e., the side farthest away from the pattern) the hardness of the granular material is not always as high as required. If, e.g., on the back side of the mold a pouring basin or gate has to be made by milling or a similar process, the hardness on this side must be incrreased substantially without any marked influence on the characteristic nature of the mold hardness.